A raw material comprising a polymer compound has characteristic gas permeability for each raw material. Capability of separation of a desired gas component is known, based on properties thereof, by a membrane constituted of a specific polymer raw material. As an industrial application embodiment of this gas separation membrane, study has been conducted for separating and recovering carbon dioxide with energy saving from a large-scale carbon dioxide generation source in a thermal power station, a cement plant, a blast furnace in a steel plant or the like in relation to a global warming issue, and attracts attention as a solution to an environmental issue. Meanwhile, natural gas or bio gas (gases generated by fermentation and anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbage's, energy crops, and the like) is mainly a mixed gas of methane and carbon dioxide. Study has been made so far for a membrane separation method as a means for removing an impurity such as carbon dioxide therein (see Patent Literature 1 and Patent Literature 2). In particular, study has been made for cellulose or polyimide as a raw material in purification of a natural gas. However, the membrane is plasticized under high pressure conditions and high carbon dioxide concentration in an actual plant, and a decrease of separation selectivity due to the plasticization has become a problem (see Non-Patent Literature 1, and Non-Patent Literatures 2 and 3). Moreover, an impurity such as a trace amount of water, hydrogen sulfide, long-chain hydrocarbon, an aromatic compound such as benzene and xylem is contained in the natural gas. Accordingly, these impurities stagnate in a separation membrane module as use is continued, and may occasionally damage a membrane raw material. From this respect also, improvement has been required for chemical stability (see Patent Literature 8).
In order to suppress plasticization of the membrane, introduction of crosslinked structure into a polymer compound constituting the membrane is known to be effective, and research has been continued for improvement in a polyimide or cellulose membrane (see Non-Patent Literature 1, and Patent Literature 4). In addition thereto, specific examples of arts utilizing a membrane having a crosslinked structure of polymide for the gas separation membrane include arts described in Patent Literature 3, and Non-Patent Literatures 4, 5 and 6.